A sensor and its evaluation circuit used to evaluate the sensor signal quite generally have interference signals that are superposed on the useful signal that is actually to be generated, i.e. the measurement signal. These include, in particular, a superposed DC signal, the offset, and the temperature dependence thereof. These influence the components of the sensor and corrupt the measurement signal and also the linearity and operating point of the evaluation elements.
Known examples of applications for sensors relate to magnetic field sensors and in particular Hall sensors comprising Hall elements. A Hall element outputs a voltage signal in the magnetic field as a Hall signal if a current flows through it perpendicular to the magnetic field. The Hall signal, that is to say the Hall voltage is dependent on the product of the vertical component of the magnetic flux density, the Hall current and the Hall constant. The Hall constant, which specifies the sensitivity of the Hall element, is material-dependent. During operation in practice, the offset composed of the offsets of the Hall element and the downstream evaluation device is superposed on the useful signal of the Hall voltage formed from the Hall constant of the component, the vertical component of the magnetic flux density and the Hall current.
U.S. Pat. No. 5,260,614 discloses a method and a magnetic field sensor with self-compensation by means of a thermal and technological coupling of the Hall element and its supply devices. For this purpose, the corresponding elements are embodied jointly in an integrated circuit. An offset voltage is superposed on the Hall voltage by means of a hysteresis circuit.
A further magnetic field sensor is disclosed in U.S. Pat. No. 5,604,433, wherein a magnetic field sensor is proposed which enables a greater accuracy by taking account of an offset of the Hall element.
U.S. Pat. No. 5,621,319 discloses a clocked Hall sensor comprising a sample-and-hold circuit and a summation element with dynamic offset suppression. It describes how the offset is eliminated by means of a summation with the aid of two signals generated from currents flowing perpendicular to one another through the Hall sensor. The method is also known as the “Spinning Current” technique. The subject matter of this document is hereby incorporated herein by reference.
US 2003/017 8989 A1 discloses a gearwheel detector with offset compensation of a magnetic field, which compensation involves recovering the sensor signal at the output of the evaluation circuit and feeding it back to the input of the evaluation circuit in order to compensate for the offset generated by the DC magnetic field. Although the detector largely eliminates said offset, the document describes that a considerable inaccuracy remains in the interference signal to useful signal ratio. The document furthermore explains that frequency discrimination does not suffice for removing the offset.
As a further example of an application, EP 0916074 B1 specifies a magnetic rotation sensor, in which a magnet mounted on an axis is arranged above a Hall element. The Hall element itself comprises a number of individual sensor elements which are in a specific geometrical arrangement with respect to one another. Each arrangement with a downstream preamplifier for the corresponding signal is referred to as a channel. The evaluation device connected downstream of the Hall element determines the rotation angle of the axis from the Hall signals of the channels. Each channel has an offset signal of the sensor arrangement and an offset of the preamplifier, to which the offset of the evaluation device is added. As a result, the actual useful signal is corrupted and an incorrect output value is determined.
A similar situation holds true if the Hall signal is digitized for the purpose of digital further processing.